This invention relates generally to an accurate, non-invasive method and apparatus for determining ocular blood flow. This invention can be used to differentiate ischemic from nonischemic oculopathies, a distinction which cannot be made always on purely clinical grounds. This invention can be used to assist is selecting those patients with compromised ocular blood flow that may benefit from angiographic evaluation and spare those patients with nonischemic disease from the risk and discomfort of angiography. Knowledge of the vascular state of the ocular fundus may contribute to an understanding of the pathophysiology of many ocular disorders and may prevent stroke and blindness by early detection of vascular disease. A person with vascular problems will have poor blood flow in the eye, and eventually diminished blood flow in the brain. Additionally, the method of this invention can potentially be a benefit in the non-invasive detection of carotid artery occlusive disease in the absence of ocular manifestations.
In general, an apparatus for fundus reflectometry consists of a light source and a photodetector, with the photodetector used to measure the reflection of light from the fundus of the eye. A dye injected into an antecubital vein flows through the retinal and choroidal vessels and is taken up preferentially in the choroidal layer causing increased absorption and therefore less reflection of light to be detected by the photodetector. The change in reflection correlates with ocular blood flow.
In the prior art, the Patent to Flower (U.S. Pat. No. 4,485,820), discloses a method and apparatus for continuously monitoring hemoglobin saturation in the blood of premature infants. A substantial portion of the eye fundus is illuminated by passing at least two frequencies of light through the pupillary opening. One of the wavelengths is the xe2x80x9cisobesticxe2x80x9d wavelength which is a wavelength at which little or no difference appears in the optical reflectance of oxyhemoglobin verses reduced hemoglobin. The invention relies on the infant eye which has a retina that is not fully vascularized and does not have significant pigmentation as in the adult eye. The light scattered from the fundus is collected as it passes out through the pupillary opening and its intensity is measured. The circulation of blood in the choroid is unique in that there is little difference between hemoglobin saturation in the choroidal capillaries and that in the choroidal arteries. Calculations known in the art are used to determine blood hemoglobin based on the intensity of scattered light. An accurate representation of oxygen saturation in arterial blood being carried to the brain can then be obtained. The patent discloses a contact lens with fiber optic links to illuminate the infant eye and collect the scattered light.
The Patent to Gardner, et al. (U.S. Pat. No. 4,346,991), discloses a method and apparatus for measuring retinal blood flow by detecting a doppler shift imparted to laser beams intersecting on a portion of a retinal vessel. Two beams of laser radiation are directed along an optical path into eye and scattered radiation is returned along the path into two photomultipliers. The beams are aligned in the eye so that the plane containing them also contains a direction of blood flow to be measured. The two beams of radiation are scattered by blood corpuscles back along the same optical path and the scattered radiation is received and processed to determine a doppler signal from which blood flow velocity in a single retinal vessel can be determined. The patent measures the blood flow velocity in a single retinal vessel and cannot be used to measure numerous vessels which would enable a closer approximation of flow volume in the eye. The retinal vessels have an autoregulatory capacity which may mask perfusion changes, so measurements of retinal flow alone may give false information regarding ocular perfusion. A more accurate estimate of ocular perfusion may be obtained by measuring choroidal flow. Furthermore, asymmetry of flow is a major component in detection of abnormal perfusion. Therefore, a device is needed that employs binocularity to simultaneously measure blood flow in both eyes.
It is therefore an object of this invention to provide a method and apparatus for binocular fundus reflectometry for use as a diagnostic tool in detecting ocular ischemia by measuring ocular blood flow. Clinical applications include the differentiation of ischemic oculopathies from inflammatory or other nonischemic disorders.
It is a further object of this invention to detect carotid artery disease in the absence of ocular manifestations.
It is a further object of this invention to provide a technique for use in the post-operative evaluation of patients undergoing revascularization procedures such as carotid endarterectomy or extracranial-intracranial bypass surgery. Patients who demonstrate no change in ocular perfusion following surgery may be suspected of having an occluded vessel at the operative site.
It is another object of this invention to obtain ocular flow measurements intraoperatively and postoperatively in the recovery room or in an intensive care unit.
It is a still further object of this invention to determine the effects of pharmocologic agents on ocular blood flow.
The apparatus for binocular fundus reflectometry is comprised of lightweight, flexible fiberoptic cables terminating in special contact lenses, a sensitive silicon chip photodetector, a specially constructed amplifier connected to a computerized dual channel strip chart recorder and an algorithm that reduces or eliminates the effects of artifactual and extraocular motion noise.
The two bifurcated fiberoptic cables, one for each eye, serve to transmit and receive light simultaneously. Use of a flexible fiberoptic cable reduces changes in lens positions produced by slight ocular movements and allows better beam collimation (focusing) than attempting to mount the light source directly on the eye. Each bifurcated cable is connected to an arc lamp which evenly illuminates the common fiber to produce nearly identical outputs at each eyepiece. The lamp is a ten watt broadband tungsten-halogen filament lamp having a maximum emission wavelength near 900 nanometers. The lamp housing intensity incident upon the fiber bundle may be varied. The maximum observed power output at the eyepiece when filtered is approximately 10 milliwatts. A DC power source and isolation transformer eliminate transient electrical fluctuations.
The two silicon photodetectors are each centrally mounted inside the housing of a differential amplifier to prevent interference from outside light. These monolithic, integrated circuits contain both a photodiode and a transimpedance preamplifier. The radiant responsivity is high with a maximum of 60 mV/W in the near infrared spectrum between 800 nanometers and 900 nanometers. Since the changes in reflection are small, the output signal is increased through a differential amplifier to a level suitable for recording on a peripheral device.